PROJECT SUMMARY/ABSTRACT Prostate cancer (PC) is the most frequently diagnosed cancer in men and one of the leading causes of mortality. PC growth and survival is driven by androgen receptor (AR) activity, a master regulator of prostate development and homeostasis. Accordingly, the frontline therapies for metastatic prostate cancer deprive AR of the activing ligands testosterone (T) dihydrotestosterone (DHT) by limiting their biosynthesis or blocking binding to the androgen receptor. However, the disease inevitably progresses to a castration-resistant, and incurable, state within 1-2 years of androgen deprivation therapy (ADT). Stemming from observations that AR signaling is dichotomous, inducing growth at lower activity levels while suppressing growth at higher levels, and that PCs eventually adapt to any static androgen environment by modulating AR activity, an effective therapeutic approach was recently devised and implemented in clinical trials termed bipolar androgen therapy (BAT). This therapy alternates between superphysiological T levels and ADT thereby suppressing castration resistant disease in a subset if patients while increasing quality of life. However, the use of T as a therapeutic is limited by concerns regarding safety and poor drug-like properties of the molecule. One potential solution is replacing T with non-steroidal AR agonists called Selective AR Modulators (SARMs), which are being developed to combat muscle wasting and cachexia. In this proposal we will evaluate the hypothesis that non- steroidal AR agonists can recapitulate the efficacy of high-T in suppressing PC growth. We will compare SARMs to T and DHT at each step in a series of molecular events leading to the activation of AR and effecting transcription: ligand binding, allosteric change, phosphorylation, dimerization, nuclear localization, chromatin binding, and finally transcription of AR target genes. Next, we will determine the mechanism or mechanisms by which SARMs, T, and DHT suppress PC by measuring a comprehensive panel of molecular readouts relating to molecular phenomenon associated with androgen-mediated anti-tumor effects including: AR splice variant repression, cell-cycle arrest, cellular senescence, DNA-damage, and repression of key oncogenes. Finally, we will directly measure the ability of SARMs to suppress the growth of PC cell lines and patient-derived xenograft models and the effect of cyclic vs continuous SARM dosing. The successful completion of our study will lead to future clinical trials utilizing SARMs as safe and effective replacements for T in BAT therapy.